Place all the herb oil ingredients in a bottle or jar with a tight-fitting lid and give it a good shake up.
This should keep for at least 3 weeks.
Preheat the oven to 220 Deg C. (425 Deg. F).
Place the roast in a roasting pan.
Paint the entire roast with the herbed oil, then place in the oven, covered, for 1 hour.
Reduce the heat to 180 Deg C. (350 Deg. F) and roast uncovered for a further 30 minutes.
Turn the roast over and roast, uncovered, for another 20 minutes.
Oven temperatures do vary, so rather under-cook than overcook the meat. It can always go back into the oven if you prefer it well done.